The manufacture of integrated circuits in a semiconductor device involves the formation of a sequence of layers that contain metal wiring. Metal interconnects and vias which form horizontal and vertical connections in the device are separated by insulating
layers or inter-level dielectric layers (ILDs) to provide electrical insulation between metal wires and prevent crosstalk between the metal wiring that can degrade device performance.
A popular method of forming an interconnect structure is a dual damascene process in which vias and trenches are filled with metal in the same step to create multi-level, high density metal interconnections needed for advanced high performance integrated circuits. The most frequently used approach is a via first process in which a via is formed in a dielectric layer and then a trench is formed above the via. Recent achievements in dual damascene processing include lowering the resistivity of the metal interconnect by switching from aluminum to copper, decreasing the size of the vias and trenches with improved lithographic materials and processes to improve speed and performance, and reducing the dielectric constant (k) of insulators or ILDs by using so-called low k materials to avoid capacitance coupling between the metal interconnects. The expression “low-k” material has evolved to characterize materials with a dielectric constant less than about 3.9. One class of low-k material that have been explored are organic low-k materials, typically having a dielectric constant of about 2.0 to about 3.8, which may offer promise for use as an ILD.
One drawback arising from the use of a copper conductor is that copper diffuses rapidly through various materials. To prevent diffusion, various materials are used as barrier materials for copper. To provide barriers between trenches and vias, the preferred barrier materials generally have been silicon nitride or silicon carbide. However, these materials have a high dielectric constant, which mean they tend to increase capacitance and thus reduce semiconductor circuit speed. However, even with the use of a barrier layer, copper is still subject to strong electro-migration, or movement of copper atoms under electrical current which can lead to formation of voids in the copper-filled trenches and vias.
In view of this problem, a barrier material is preferred that prevents copper diffusion, reduces copper electro-migration and has a lower dielectric constant. One such material that has been proposed which largely satisfies these criteria is the metal alloy cobalt tungsten phosphate (CoWP). CoWP also has the advantage that it can be selectively formed only on the copper layers by electroless plating.
Despite the use of CoWP as a barrier material, an additional cap layer is generally also required to serve as a hardmask when a second interconnection (i.e., a trench or via) is etched in an ILD over a first interconnection (i.e., a trench or via). Without the additional cap layer, so-called micro-trenchess may be formed in the ILD as a result of lithographic misalignment. Unfortunately, the cap layer, which is typically silicon nitride or silicon carbide, undesirably increases the dielectric constant of the structure.
Accordingly, it would be desirable to provide a dual damascene interconnect in which the cap layer can be reduced in thickness when a barrier layer is employed between multilayer metal interconnections.